The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of up to 12, typically 2, 3 or 4, but up to 5, 6, 7, or even 8 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butenes, pentenes and hexenes) can be converted by oligomerisation over a solid phosphoric acid catalyst, to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include alcohols, acids, detergents and esters such as plasticiser esters and synthetic lubricants. Industrial oligomerisation reactions are generally performed in a plurality of tubular or chamber reactors. Sulfated zirconia, liquid phosphoric acid and sulfuric acid are also known catalysts for oligomerisation.
Industrial hydrocarbon conversion processes employing acidic catalysts such as those mentioned above typically run for several weeks before a catalyst change is required or a decommissioning of the reactor is needed. In industrial processes the feeds for the reactions are generally obtained from refining activities such as a stream derived from catalytic or steam cracking, which may have been subjected to fractionation. The nature of such refining activities is such that there will be variations in the composition of the feed. In addition it may be desired to change the nature of the feed during a reactor run. The catalyst activity and the reaction conditions vary according to the composition of the feed. Furthermore, the reactions are exothermic and the size of the exotherm also depends upon the nature and amount of olefin present in the feed. Isobutylene and propylene are particularly reactive over solid phosphoric acid catalysts, generating a large amount of heat per unit of mass reacting.
Olefin oligomerization using solid phosphoric acid catalyst may be performed using chamber type reactors, comprising a plurality of catalyst beds sequentially located within one reactor shell and each bed acting as an adiabatic reactor. Due to the reaction heat, the reacting mixture heats up as it passes through a catalyst bed. In between two catalyst beds, a colder diluent stream may be injected to bring the temperature down again before the mixture of the reacting fluid and the diluent enters the following bed. The injection of the colder diluent stream into the reactor is an important element. It is typically preferred to have several spray nozzles at each point of injection, distributed over the reactor cross section such that a maximum of the cross sectional area is covered, and with the spray nozzles preferably but not essentially pointed in the direction against the flow of the process fluid in the reactor. One option is to arrange 4 quench nozzles, approximately 90° apart such as in a cross configuration, each of the nozzles having a full cone spray pattern with a 164° spray angle. An alternative, preferable for a reactor with a smaller diameter, is a simple quench nozzle in the middle, having a hollow cone spray pattern and a 140° spray angle. The hollow cone nozzles with a wide angle are found to cover a wider area, and are therefore preferred. Below and downstream of this quench nozzle arrangement, especially suitable for reactors with a larger diameter and in particular at those locations where 2-phase flow may occur such as in the bottom section of a chamber reactor, a distributor tray may be added, equipped with slotted chimneys and extra tubes depending on the vapor load. All these reactor internals are preferably such that they may be disassembled and removed to provide access to the catalyst beds for removing and replacing the catalyst, after which they may be introduced and assembled again. The top of the catalyst bed may then be further protected against liquid impingement by a layer of inert material, e.g. ceramic balls. This setup provides a better mixing in a minimum volume, thereby reducing the risk for radial temperature and concentration differences and for hot spots in the downstream catalyst bed.
The present invention is concerned with oligomerisation processes that employ a solid phosphoric acid “SPA” oligomerisation catalyst in a tubular reactor.
Tubular oligomerisation reactors employing SPA catalysts typically comprise one or more bundles of tubes, also termed “reactor tubes”, mounted, preferably vertically, within a shell, and may be similar to a vertical shell-and-tube heat exchanger. The reactor tubes are packed with the SPA catalyst typically in the form of pellets and the olefin reactant is passed through the tubes in which it is oligomerised, typically from top to bottom. The length of the tube in industrial practice is generally from 2 to 15 meters, preferably from 7 to 12 meters. The diameter of the tube, the thickness of the walls of the tubes and the materials from which the tubes are made are important since oligomerisation reactions are exothermic and it is important to dissipate the heat generated by the oligomerisation reaction. Accordingly relatively small diameter, such as from 3 to 10 cm tubes, are preferred, more preferably 4 to 6 cm diameter tubes. They are preferably of high strength material and are thin walled and of a material with a high thermal conductivity. The high strength is required to withstand the high pressures that are generally used in the oligomerisation of olefins in a tubular reactor employing a SPA catalyst. Duplex stainless steel is a preferred material for manufacture of the tubes.
Any convenient number of tubes may be employed in a reactor shell. Typically, operators use from 10 to 500 tubes per shell, preferably arrayed in parallel. Preferred reactors contain about 77 tubes or 180 tubes per shell, although any number may be employed to suit the needs of the operator, eg 360 or 420. The tubes are preferably mounted within the shell and a temperature control fluid is provided around the outside of the tubes but within the shell to dissipate heat generated by the exothermic reaction that, in use, takes place within the reactor tubes. One reactor may comprise multiple bundles of tubes, for example up to 7 or 8, or even 9 bundles, and preferably, in use, the temperature of the fluid within the tubes in all the bundles in the same reactor is controlled by means of the same temperature control fluid system.
Reference in this specification to removal of heat from the (reactor) tubes or temperature control of the (reactor) tubes is, in context, intended to mean removal of heat from the materials contained within the tubes where reaction takes place (generally comprising, in use, unreacted feed, reaction products and catalyst) and control of the temperature of those materials contained within the tubes. It will be appreciated that the heat generation on the catalyst and heat removal from the tube wall may cause a radial temperature gradient through the cross-section of the tube, such that the center of the tube may become significantly hotter than the wall of the tube. One convenient way to remove the heat from the tubes and carry out the temperature control is to generate steam within the reactor on the shell side around the exterior of the tubes. This provides a good heat transfer coefficient on the shell side. If the present invention is performed in a chemical plant or a refinery the steam generated by the oligomerisation process may be readily integrated into the steam system typically present at such sites. The reaction heat from oligomerisation may then be put to use in another part of the oligomerisation process, or with another process in the plant or the refinery, where heat input is required.
On an industrial scale it is desirable that these tubular reactors can run continuously for as long as possible and that the conversion and selectivity of the reaction is maintained over such extended production runs.
U.S. Pat. No. 6,884,914 relates to the oligomerisation of olefins and provides an olefin feed stream which can be oligomerised at high efficiency. The olefin feed stream may be obtained from oxygenates by treatment with mole sieves. However, refining feeds may also be used, though these are preferably used in admixture with the olefin feed obtained from the oxygenates. The feed preferably contains about 55 wt % olefin and more preferably 60 wt % olefins. U.S. Pat. No. 6,884,914 discusses various different catalysts that can be employed including solid phosphoric acid, although zeolite catalysts are preferred. Here the oligomerisation reaction is performed at a temperature from 170° C. to about 300° C., preferably about 170° C. to 260° C., most preferably about 180° C. to about 260° C. Operating pressure is said to be not critical although the process is carried out at about 5 MPa to 10 MPa. In the oligomerisations exemplified in U.S. Pat. No. 6,884,914, a feed containing 64 wt % butenes is oligomerised using a ZSM-22 zeolite catalyst.
U.S. Pat. No. 6,884,914 is not, however, concerned with optimising oligomerisation of olefins in a tubular reactor employing SPA catalyst. Tubular reactors are the most efficient for oligomerisation reactions over SPA catalyst because the reactions are highly exothermic and require precise temperature control.
As already indicated, the oligomerisation of olefins over a SPA catalyst is a highly exothermic reaction, particularly the oligomerisation of propylene and/or isobutylene. The high temperatures generated can lead to carbonaceous deposits on the catalyst caused by a build up of condensed, heavy hydrocarbons similar to asphalt. Such deposits are commonly termed “coke”, and lead to deactivation of the SPA catalyst. In general, the higher the concentration of olefin in the feed, the higher will be the rate of heat release from the catalysed reaction, and hence the higher the temperatures reached. Consequently there will be a higher rate of coke formation. This has placed a limit on the maximum concentration of olefin that can be tolerated in the feed.
The composition of the material in the tubular reactor varies as the material flows through, usually down, the reactor tube and begins to react. The olefin will have a lower molecular weight at the beginning (inlet) of the reactor tube, where it is predominantly unreacted light olefins and it will become progressively heavier towards the tube outlet as the light olefins are oligomerised to form higher molecular weight olefins.
Oligomerisation over a fixed bed of SPA catalyst may show a higher exotherm than over a zeolite catalyst such as ZSM-22. Excessive temperatures coke up the SPA catalyst, which makes it swell. Furthermore, coke also collects in the volume between the catalyst particles. Pressure drop over the SPA catalyst bed then increases to the point that the reactor has to be taken out of service. The spent SPA catalyst bed then has to be drilled out and typically be disposed of as solid waste. A high pressure water lance can be used as an alternative to drilling. However, this produces a waste sludge which is even more difficult to dispose of. The reactor tubes then have to be filled with fresh SPA catalyst and the reactor may then be put into oligomerisation service again.
In the operation of a tubular reactor for oligomerisation of olefin feed, with SPA catalyst in the tubes and a temperature control fluid on the shell side, a temperature profile will be observed over the length of a reactor tube. Conventionally, such operation is performed with the tubular reactor arranged such that the feed inlet is at the top and the reaction product outlet is at the bottom. The following description addresses such an arrangement, but it will be understood that the description applies equally to reactors not in top to bottom arrangement. Thus, the temperature profile initially shows a sharp increase at the inlet of the tube, when reaction heat is generated faster than it can be removed by the temperature control fluid around the tube. As the reactants convert further as they move along the tube and their concentration reduces, the reaction rate reduces and the rate of heat generation reduces. At the same time the temperature in the tube increases, and the heat removal rate increases. The temperature profile then typically goes through a maximum, and then shows a decline further along (down) the tube towards the outlet. As the reaction temperature declines along the tube, also heat removal rate reduces, and the temperature profile may then flatten out before the end of the catalyst bed in the tubes is reached.
With fresh catalyst, the temperature increase at the initial part (eg top) of the tube is sharp, and the temperature profile shows a sharp peak. The fresh catalyst at the initial part (top) of the tube performs most of the reaction. Coke will build up where the temperature is at its highest, which will deactivate the catalyst in that part of the tube. Reaction rate will then reduce in that part of the tube due to the catalyst deactivation, and hence the rate of heat generation will also reduce, and hence the slope of the temperature increase in that part of the temperature profile declines. The catalyst further along (down) the tube will then see a higher concentration of unreacted reactants, and the reaction rate—and hence heat generation rate—will increase in that part of the tube. In this way the peak in the temperature profile known as “the peak temperature” will move along (down) the tube. In order to compensate for the reduced overall catalyst activity, cooling rate is typically reduced by increasing the temperature of the temperature control fluid around the tube. The average temperature in the reactor and the temperature at the outlet of the tube or reactor will thereby be increased. In addition, the inlet temperature to the tube may be adapted as well. Typically it may be increased to keep as much of the reaction as possible at as early (high) as possible a location in the catalyst bed inside the tube. The peak in the temperature profile therefore may not only move along (down) the tube as a production run proceeds but it may also become less sharp and less pronounced.
The rate of heat generation increases with higher reactant concentration. The peak in the temperature profile is therefore sharper and more pronounced when the olefin concentration in the feed to the reactor is higher. The rate of heat generation is also higher with more reactive reactants, typically with the lighter olefins such as propylene and butenes such as isobutylene. The peak in the temperature profile is therefore also sharper and more pronounced when a higher portion of the available butenes is isobutylene, or when a higher proportion of the olefins fed to the reactor is propylene. In case dienes or acetylenes are present, these are even more reactive and will increase the rate of heat generation, in particular in the upstream part of the SPA catalyst bed. The total heat of the reaction also depends on the product produced. The greater the degree of oligomerisation of any particular olefin the higher the heat of reaction, because more monomer molecules will have combined to form the product.
The level of di- and polyunsaturates in the feed is typically controlled to below a maximum allowable level. Preferably, the feed composition is limited to containing no more than 100 ppm by weight of acetylene and/or no more than 500 ppm of the C3 polyunsaturates methylacetylene and propadiene or allene, and/or no more than 2500 ppm or more preferably no more than 1000 ppm of butadiene. The reason for these limitations is the high reactivity and the high heat of reaction of the di- and polyunsaturates relative to their molecular weight. Pentadiene has a heat of reaction that is almost the same as propylene. We have also found that cyclopentene generates substantially the same heat of reaction as pentadiene. We have found that if it is necessary to use feeds containing relatively high levels of polyunsaturates, production may be sustained if the olefin concentration in the feed is reduced accordingly. This keeps the carbon deposition low which would otherwise increase due to the heat generated by the reaction of the higher amounts of polyunsaturates present.
The olefin feed to the tubular reactor is generally a mixture of a reactive olefin and an unreactive diluent, which is typically an alkane. This may have the same carbon number as the olefin. However, it is preferred to have unreactive components present that have a higher carbon number than the feed olefin because of their advantageous effect on phase behaviour in the reactor. The rate of heat generated by the oligomerisation reaction depends upon the concentration of the olefin in the feed. The higher the concentration of olefin the more reactive the feed and the greater the heat that is generated. For example in the operation of tubular reactors employing SPA catalysts to oligomerise propylene containing feeds it has been found necessary to limit the amount of olefin in the feed. This is because, despite employing cooling systems such as the steam generation mentioned previously, it has not been possible to perform extended continuous runs with feeds containing more than 50 wt % propylene. Typically it has only been possible to employ feeds containing much less than 50 wt % propylene, some processes operating at 40 wt % propylene or less.
The feed streams containing the feed olefins such as C3 and C4 olefins are generally refinery steams derived from steam cracking or catalytic cracking, and the composition of the stream will depend upon the raw material from which it is produced and the production technology employed. However, propylene refinery streams typically contain up to 75 wt % propylene with the balance being predominantly propane. Similarly butene refinery streams typically contain up to 70 wt % butenes with the balance being predominantly butanes. The reactivity of the olefins in oligomerisations over SPA catalysts varies according to the nature of the olefin. However it has not been possible to successfully oligomerise C3 to C6 olefins over extended periods of time in tubular reactors employing a SPA catalyst if the concentration of propylene in the feed exceeds 50 wt %, and generally concentrations below 40 wt % have been employed. This has required the expensive addition of diluent to an olefin-containing refinery feed. Typically the diluent may be additional amounts of the alkanes found in the refinery feed and/or it maybe provided by recycle of the unreacted material derived from the outlet of the tubular reactor. The need for diluent not only adds to the expense of the operation but it also reduces the volumetric yield of the reaction, with associated economic debits.
In our Patent Application WO 2005/058777, we describe phosphoric acid catalysts and how they may be employed in the oligomerisation of olefins. In particular we describe how the SPA catalyst may be hydrated to improve catalyst performance. These features apply equally to the present invention, and to the porosity profile of the catalyst pellets.
SPA catalysts are typically prepared by combining a phosphoric acid with a support and drying the resulting material. A commonly used catalyst is prepared by mixing kieselguhr with phosphoric acid, extruding the resulting paste, and calcining the extruded material. The activity of a SPA catalyst is related to the amount and the chemical composition of the phosphoric acid which is deposited on the support and to the porosity profile of the catalyst pellets.
Phosphoric acid comprises a family of acids, which exist in equilibrium with each other and differ from each other in their degree of condensation. The catalysts are generally supported on silica and consist of silicon phosphate crystals coated with various phosphoric acids. These acids include ortho-phosphoric acid (H3PO4), pyro-phosphoric acid (H4P2O7), triphosphoric acid (H3P3O10), and polyphosphoric acids, and the precise composition of a given sample of phosphoric acid will be a function of the P2O5 and of the water content of the sample. As the water content of the acid decreases the degree of condensation of the acid increases. Each of the various phosphoric acids has a unique acid strength and accordingly the catalytic activity of a given sample of SPA catalyst will depend on the P2O5/H2O ratio of the phosphoric acid which is deposited on the surface of the crystals.
One factor that influences the activity of a SPA catalyst and also its rate of deactivation in an oligomerisation process, is the degree of catalyst hydration. A properly hydrated SPA catalyst can be used to convert over 95% of the olefins in a feedstock to higher molecular weight oligomers. However, if the catalyst contains too little water, it tends to have a very high acidity, which can lead to rapid deactivation as a consequence of coking. Further hydration of the catalyst serves to reduce its acidity and reduces its tendency toward rapid deactivation through coke formation. On the other hand, excessive hydration of a SPA catalyst can cause a change in the crystal structure, leading to lower density and swelling. This change may cause the catalyst to soften and physically agglomerate and, as a consequence, can create high pressure drops in the tubular reactors. Accordingly, there is an optimum level of hydration for a SPA catalyst. In our Patent Application WO 2005/058777, we describe our preferred means of ensuring optimum hydration of SPA catalyst.
During use as an oligomerisation catalyst, a SPA catalyst will develop a degree of hydration which is a function of feedstock composition and reaction conditions. For example the level of hydration is affected by the water content of the feedstock which is being contacted with the catalyst and also by the temperature and pressure at which the catalyst is used. The equilibrium vapour pressure of water over a SPA catalyst in a particular hydration state varies with temperature and it is important to keep the water content of the feedstock at the proper concentration to maintain optimal catalyst hydration and acidity. If a substantially anhydrous hydrocarbon feedstock is introduced with a properly hydrated catalyst, the catalyst will typically lose water during its further use, and will develop a less than optimal degree of hydration. Accordingly when the water content of a feedstock is inadequate to maintain an optimal level of catalyst hydration, it has been conventional to inject additional water into the feedstock. A study of the effect of water on the performance of SPA catalysts as catalysts for the alkylation of benzene with propene and for the oligomerisation of propene is set forth in a review article by Cavani et al, Applied Catalysis A: General, 97, pp. 177-1196 (1993).
As an alternative to incorporating water into a feedstock that is being contacted with a SPA catalyst, it has also been proposed to add a small amount of an alcohol, such as 2-propanol, to the feedstock, to maintain the catalyst at a satisfactory level of hydration. For example, U.S. Pat. No. 4,334,118 discloses that in the polymerisation of C3-C12 olefins over a SPA catalyst which has a siliceous support, the catalyst activity can be maintained at a desirable level by including a minor amount of an alkanol in the olefin feedstock. It is stated that the alcohol undergoes dehydration upon contact with the catalyst, and that the resulting water then acts to maintain the catalyst hydration level.
It is also known from, for example, U.S. Pat. Nos. 4,334,118 and 5,744,679 that by using in an alkene oligomerisation process an alkene-containing feedstock with a water content of from 0.05 to 0.25 mol %, and preferably of at least 0.06 mol %, based on the hydrocarbon content of the feedstock, the catalyst becomes deactivated more slowly.
It is also known that when an alkene-containing feedstock has a water content of less than 0.05 mol %, the water content may be increased by a variety of means. For example, the feedstock can be passed through a thermostatic water saturator. Since the amount of water required to saturate the alkene feedstock depends upon the temperature of the feedstock, control of the water content can then be effected by appropriate control of the temperature of the feedstock. The water content of the feedstock is preferably at least 0.06 mol %, based on the hydrocarbon content of the feedstock.
The quantity of water included must be sufficient to accomplish the appropriate hydration of the SPA catalyst in order to provide and sustain the desired catalytic activity. However, as indicated, too much water can cause a swelling of the catalyst leading to deactivation. In some instances an olefin feed to the oligomerisation reactor may contain materials that can decompose on contact with the SPA to form water. For example, a propylene containing stream that is derived from the industrial production of isopropanol may be used as a component of the feed stream for oligomerisation. However, such a stream will generally contain di-isopropyl ether, which, is decomposed by contact with a SPA catalyst to produce propylene and isopropanol, and the isopropanol is decomposed again to propylene and water. The feed may, however, already be saturated with water as a result of the water washing of the feed and typically that water of saturation will be sufficient to optimise the activity of the SPA catalyst. Therefore, the additional water produced by the decomposition of the di-isopropyl ether can result in excess catalyst hydration, and hence earlier catalyst failure.
Therefore, there remains a need to oligomerise olefin feeds containing a higher concentration of olefin using a SPA catalyst over extended production runs in a tubular reactor without undue deactivation or premature failure of the SPA catalyst. There also remains a need to oligomerise feeds containing compounds that generate water on contact with the SPA catalyst over extended production runs without undue deactivation or failure of the catalyst. It will be appreciated that in large scale industrial processes such as are used for the oligomerisation of olefins, small increases in production (such as 1 to 5% increase) have highly significant benefits. In addition, the ability to increase a run length by an apparently small amount also has highly significant benefits.
We have now developed an oligomerisation process that is capable of providing such benefits.
We have found that, regardless of what the overall, average or outlet temperatures of the reactor may be, if the peak temperature is allowed to reach too high a level, the catalyst deactivation rate and the coke build up rate becomes excessive, the catalyst swelling becomes excessive, and the life of the catalyst bed is reduced. We have also found that control of the peak temperature enables water-producing feeds to be processed without the water unduly deactivating the catalyst.
We have found that in order to obtain good catalyst life in an oligomerisation process comprising a tubular reactor containing SPA catalyst, it is important to control the peak temperature at all times and this is more important than to control the overall or average or outlet temperature of the reactor.